Recently, electromagnetic waves in the THz frequency band between light and radio waves (namely the electromagnetic waves at a frequency of 1012 Hz and in a wavelength range approximately from 30 μm to 1 mm, which are termed the THz waves hereafter) have drawn attention as electromagnetic waves directly reflecting substance information. Techniques of bolometer-type infrared detectors having thermal isolation structures are applied to detection of the THz waves. Bolometer-type THz wave detectors are among the related art and described in Unexamined Japanese Patent Application KOKAI Publication No. 2008-241438 and a SPIE Paper (Oda et al., Proceedings of SPIE, Vol. 6940 (2008) pp. 69402Y-1 to 69402Y-12). There are many bolometer-type infrared detector-related techniques. Among them, technical matters relating to the thermal isolation structure, which is in frequent use, are described in U.S. Pat. No. 5,286,976 and a SPIE Paper (J-L. Tissot et al., Proceedings of SPIE, Vol. 3379 (1998) pp. 139-144).
However, in the pixel structure of related art in Unexamined Japanese Patent Application KOKAI Publication No. 2008-241438, since the distance between the reflective film and temperature detecting unit is set based on the infrared wavelengths and the sheet resistance of the temperature detecting unit is set based on the THz waves, the thermal isolation structure having an absorbing film has an absorptance of approximately 18 to 40% for a wavelength of 100 μm. For other wavelengths, the formula in the SPIE Paper (Oda et al., Proceedings of SPIE, Vol. 6940 (2008) pp. 69402Y-1 to 69402Y-12) can be used to calculate the wavelength properties of the absorptance. The results obtained for an air gap of 1.5 μm (FIG. 10) and for an air gap of 2.5 μm (FIG. 11) are shown. As shown in FIG. 10 and FIG. 11, the absorptance is not really high for almost the entire THz wave range. For example, the absorptance is 5 to 10% for a wavelength of 300 μm.
In the pixel structures of other related art (U.S. Pat. No. 5,286,976 and SPIE Paper (J-L. Tissot et al., Proceedings of SPIE, Vol. 3379 (1998) pp. 139-144)), for instance, the air gap height has to be 25 μm in order to maximize the absorptance for an electromagnetic wave of 3 THz (100 μm in wavelength). By applying the air gap height to the formula in the SPIE Paper (Oda et al., Proceedings of SPIE, Vol. 6940 (2008) pp. 69402Y-1 to 69402Y-12), the absorption properties shown in FIG. 12 are obtained. As shown in FIG. 12, the absorptance is improved; however the absorptance around a wavelength of 50 μm is low. Then, it is difficult to realize a THz wave detector that does not lose or extremely diminish the sensitivity around a specific wavelength among the shorter wavelengths including 30 μm or longer in the THz wave range.